feat(sky): replace the blocky default sky box with catalogue stars

Cesium's default Tycho-2 sky box is six 1024² JPEGs stretched 3–6× around
the whole-globe view, so space read as an over-compressed JPEG (HAL-627).

The scene now draws its own sky in the Scene#skyBox slot: the 9,096 stars
of the Yale Bright Star Catalogue as point sprites sized in CSS pixels
(magnitude → width and opacity, B−V → colour), over a soft, star-free
Milky Way cube map built from ESO/S. Brunier's panorama (CC BY 4.0). Both
layers share one ICRF→Earth-fixed rotation at the scene clock, with the
TEME rotation filling in until Cesium's IAU 2006 tables load.

Assets total 482 KB, built by scripts/build-bright-stars.mjs and
scripts/build-milky-way.mjs. Frame time and the idle render governor are
unchanged.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/data/local_data/sky/README.md

@@ -0,0 +1,65 @@
+# Sky background
+
+The space behind the globe (`src/sky/starrySky.js`): catalogue stars drawn as
+sharp point sprites over a soft Milky Way cube map. It replaces Cesium's
+default Tycho-2 sky box, whose six 1024² JPEGs stretch 3–6× around the
+whole-globe view and show compression blocks.
+
+| File | Content | Size |
+| ---- | ------- | ---- |
+| `bright_stars.json` | 9,096 stars: J2000 RA/Dec, V magnitude, B−V | 235 KB |
+| `milkyway_{px,nx,py,ny,pz,nz}.webp` | Milky Way glow, six 1024² cube faces, no stars | 247 KB |
+
+## Stars — Yale Bright Star Catalogue
+
+**Source:** Yale Bright Star Catalogue, 5th Revised Edition (Hoffleit &
+Warren 1991), CDS VizieR catalogue
+[V/50](https://cdsarc.cds.unistra.fr/viz-bin/cat/V/50), file
+`https://cdsarc.cds.unistra.fr/ftp/V/50/catalog.gz` (sha256
+`3dc44b1e90be8fbe5bcc7656032560f51275f985c7e3f783c9028e1838ec7bed`, fetched
+2026-10-19).
+
+**License:** public domain (Yale University Observatory / NASA Astronomical
+Data Center). CDS asks for an acknowledgement; the in-app "Data attribution"
+popover carries it.
+
+**Build:** `node scripts/build-bright-stars.mjs [catalog.gz]` — every record
+with a J2000 position (14 novae and non-stellar entries dropped), RA/Dec
+rounded to 0.001°, magnitudes to 0.01. The renderer maps magnitude to sprite
+width and opacity and B−V to a desaturated black-body colour
+(`src/sky/stars.js`).
+
+## Milky Way — ESO/S. Brunier
+
+**Source:** "The Milky Way panorama", ESO image
+[eso0932a](https://www.eso.org/public/images/eso0932a/), full-size original
+`https://cdn.eso.org/images/original/eso0932a.tif` (6000×3000 sRGB TIFF, sha256
+`10f209ab83e1fd89e7fa1ed70277ffc6ed19c43549f04ec637c6806d98aff035`, fetched
+2026-10-19). Plate carrée in galactic coordinates, centred on the Galactic
+Centre.
+
+**License:** [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/)
+(<https://www.eso.org/public/outreach/copyright/>). **Credit: ESO/S. Brunier.**
+
+**Changes made** (CC BY 4.0 asks for them to be stated): resampled to
+4096×2048, two planets (Jupiter, by position) painted over with the surrounding sky,
+stars removed with a 7×7 morphological opening, blurred (σ = 0.4°),
+reprojected from galactic coordinates to ICRF cube faces, black level set at
+the darkest 25 % of the sky, gamma-encoded and saved as WebP. Build:
+`node scripts/build-milky-way.mjs <eso0932a.tif>`.
+
+The panorama is a photographic mosaic, not an astrometric map: bright stars
+sit up to ~2° from their catalogue positions. That is invisible in a glow
+blurred this much, and every sharp star on screen comes from the catalogue.
+
+The NASA SVS "Deep Star Maps 2020" Milky Way layer was considered and not
+used: it is derived from Gaia DR2, which ESA distributes under CC BY-NC 3.0
+IGO with prior authorization required for commercial use.
+
+## Frame
+
+Both layers are in ICRF/J2000. The cube faces use the standard WebGL face
+layout with +X towards the March equinox and +Z towards the north celestial
+pole, uploaded without `flipY`. At runtime one rotation —
+`Transforms.computeIcrfToFixedMatrix` at the scene clock — turns both into
+Earth-fixed coordinates.